Most modern receivers employ electronic tuning instead of mechanical tuning, which requires mechanical linkage between the tuning control on the receiver, the local oscillator, the resonant antenna and the RF (Radio Frequency) circuits. Typical modern receivers with electronic tuning include front end circuitry, which provides amplification and frequency selectivity to the RF signal prior to the mixer. The front end circuitry also generates an oscillator signal that has a frequency equal to a fixed Intermediate Frequency (IF) plus the desired FM channel frequency.
The RF and oscillator signal are input to the mixer that translates the desired FM channel signal to the fixed IF signal, which is typically 10.7 MHz. The remainder of the amplification and frequency selectivity prior to the FM detector occurs at this fixed IF.
FIG. 1 shows a basic front end block diagram of a typical electronically tuned FM receiver. The circuitry indicated in FIG. 1 is fairly typical implementation.
FIG. 2 illustrates a basic block diagram of PLL (phase lock loop) 51 comprising frequency synthesizer 50 (also shown in FIG. 1) and VCO (voltage controlled oscillator) 40.
Voltage controlled oscillators such as VCO 40 are well known to those skilled in the art. In general VCO 40 comprises active semiconductor devices and an LC tank, which controls the frequency of oscillation. The exact circuit configuration of the VCO will not be discussed as a variety of circuits known to those skilled in the art can be used.
The majority of the capacitance in the LC tank is achieved using a varactor, which is a semiconductor device with a variable capacitance that is an exponential function of a DC voltage applied to the device. The frequency of VCO 40 is varied by changing the tuning voltage applied to the varactor, which changes its capacitance.
Frequency synthesizer 50 generally comprises amplifier 52, programmable divide by N frequency divider 60, serial port 62, crystal oscillator 68, programmable divide by R frequency divider 70, phase comparator 64 and loop filter 66. The components of the frequency synthesizer are typically included on one IC, but may alternatively be included in a microcomputer IC.
The operating frequency of PLL 51 is controlled by the receiver microcomputer 48 via serial port 62, through which microcomputer-generated control signals program frequency dividers 60 and 70. Crystal oscillator 68 typically operates at a few MHz and provides a crystal reference frequency signal to the input of frequency divider 70. Frequency divider 70 divides the crystal reference frequency by an integer, R, determined by the control signal generated by the microcomputer, to produce the phase comparison frequency. The phase comparison frequency is preferably chosen such that the FM channel spacing is an integer multiple of the phase comparison frequency. The output of frequency divider 70 is connected to one input of the phase comparator 64.
VCO 40 is connected to the input of amplifier 52. Amplifier 52 increases the signal level of the signal output by VCO 40 and provides the resultant signal to the input of frequency divider 60. Frequency divider 60 divides the frequency of VCO 40 by an integer, N, and provides the output to a second input of phase comparator 64.
Phase comparator 64 compares the phases of the signals from frequency divider 60 and frequency divider 70 and produces an output signal proportional to the phase difference between the signals. The output of the phase comparator 64 represents the loop error of the PLL and is connected to the input of loop filter 66.
Loop filter 66 is of a type well known to those skilled in the art and has one of a variety of possible filter transfer functions. The transfer function must yield a stable loop and can generally be varied significantly by those skilled in the art depending on tradeoffs between various loop performance parameters.
The output of the filter 66 is connected to the tuning voltage control of VCO 40 to complete the control loop.
The tuning voltage output of filter 66 forces VCO 40 to change frequency until the phase error at the output of phase comparator 64 is zero. At this point, the loop 51 is locked and frequency output of VCO 40 is equal to N times the phase comparison frequency, which is directly related to the crystal reference frequency, providing high accuracy to the frequency of VCO 40.
Referring again to FIG. 1, the receiver is tuned to a particular FM station via the front panel controls (not shown). These controls are connected as inputs to the receiver microcomputer 48. Microcomputer 48 determines the desired FM channel frequency and sends a serial data string via the IRB (Internal Radio Bus) 49 to the frequency synthesizer 50. The serial data string received by frequency synthesizer 50 programs PLL 51, comprising VCO 40 and frequency synthesizer 50, to the proper frequency as described previously. The output frequency of VCO 40 will be equal to the desired station frequency plus the IF (10.7 MHz). The output of VCO 40 is connected to one input of mixer 34 as shown.
Antenna 12 acts as an RF signal source by receiving a broad range of RF signals including the FM broadcast band and providing the RF signals to the input of antenna tank 14, which is a parallel resonant LC circuit. Antenna tank 14 comprises capacitor 16, inductor 18 and varactor 20 and acts as a bandpass filter having a band pass frequency responsive to the tuning voltage of varactor 20. Inductor 18 is of a type known to those skilled in the art that can be adjusted by mechanically moving the ferrite core. The capacitance of antenna tank 14 is largely determined by varactor 20.
The output of antenna tank 14 is connected to the input of RF amplifier 24. The output of RF amplifier 24 is connected to RF tank 26, which is another band pass filter similar to antenna tank 14 and comprises capacitor 28, inductor 30 and varactor 32. RF tank 26 has a band pass frequency responsive to the tuning voltage of varactor 32. The output of RF tank 26 is connected to the other input of mixer 34. The function of antenna tank 14, RF amplifier 24, and RF tank 26 is to selectively amplify a narrow band of frequencies centered about the desired FM station. This action limits the total RF energy into mixer 34, and prevents image signals from being translated into the IF bandwidth.
The tuning voltage from frequency synthesizer 50 is connected to varactor 46 in VCO 40, and is also connected to varactors 20 and 32 in antenna tank 14 and RF tank 26. Thus, as the tuning voltage output of VCO 40 is changed in order to change the frequency of the VCO 40, the center frequencies of antenna tank 14 and RF tank 26 are also changed. The three varactors 46, 20 and 32, used in VCO 40, antenna tank 14, and RF tank 26, are a matched set having similar voltage versus capacitance characteristics.
If the inductors in these three circuits 14, 26 and 40 are adjusted properly, then the center frequencies of antenna tank 14 and RF tank 26 will equal the output frequency of VCO 40 frequency minus the IF (10.7 MHz), which would also be equal to the desired FM station frequency, and will track the output frequency of VCO 40 as it is tuned across the FM band.
During manufacture of the receiver, inductors 18 and 30 are adjusted to align the receiver in the following manner. First, a signal generator is connected to the receiver at the antenna input. Then the receiver and the signal generator are programmed to a given frequency--generally somewhere near the lower end of the FM band. Inductor 44 in VCO 40 is then mechanically adjusted so that varactor 46 has sufficient tuning range to cover the FM band--typically a tuning range of several volts is required. In practice, inductor 44 is varied until the tuning voltage is equal to a predetermined value at a given output frequency of VCO 40.
The amplitude of the signal generator is set to a value where the receiver AGC (automatic gain control) curve has significant slope. The AGC is a function common to essentially all FM receivers. The detector for this function is located after and/or in the IF amplifier (not shown) of the receiver. The AGC voltage generated by this detector is a function of the on channel carrier amplitude. The AGC curve is generally not completely linear, but should be monotonic, and may have a positive or negative slope depending on the sense of the gain control. The AGC voltage is used to adjust the gain in the RF amplifier and perhaps other stages, as a function of signal strength. The important point is that the AGC voltage is an indicator of signal strength through the IF.
With a known signal frequency and amplitude into the receiver input, inductors 18 and 30 in antenna tank 14 and RF tank 26 are adjusted by mechanically moving the ferrite cores so that the signal through the IF is maximized, as indicated by the AGC voltage.
Antenna tank 14 and RF tank 26 are thus aligned such that their center frequencies are correct at one frequency in the FM band. This type design then depends on the varactor characteristics matching well enough that the center frequencies of antenna tank 14 and RF tank 26 will properly track VCO 40 as it is tuned across the band. In practice, the bandwidth of antenna tank 14 and RF tank 26 are wide enough that the tracking errors do not cause serious receiver performance problems.
The mechanical alignment of the inductors is generally accomplished in one of two ways. Manufacturing personnel can physically adjust the inductors by hand using a small hand tool, or the inductors can be adjusted at an automatic alignment station. An automatic alignment station is fairly sophisticated electronic/mechanical apparatus that locates and rotates the inductor cores, while monitoring the necessary electronic signals.
There are some performance limitations of the system described above, including some error in tracking due to the IF offset and varactor mismatch. VCO 40 must be tuned from 98.8 to 118.6 MHz to cover the FM band, while antenna tank 14 and RF tank 26 must be tuned from 88.1 to 107.9 MHz. While the tuning range is 19.8 MHz in both cases, the percent change is different. Since the tuning voltage applied to the varactors is identical, this creates some error as the receiver is tuned far away from the alignment point.
There is also some mismatch between the varactors 20, 32 and 46, even though matched sets are used. Each set of varactors is taken from one classification, but each classification represents a range of capacitance versus voltage characteristics. Therefore all varactors within a set are not identical. The tracking can be improved somewhat by adding more adjustable components to the tank circuits, and aligning at more points on the band, but such designs and procedures yield only marginal improvements in tracking. Therefore, design techniques similar to those described are usually used.
The tracking errors described above cause the front end band pass filter center frequencies to vary from the desired station frequency across the FM band. This will cause some loss of sensitivity and increase the total harmonic distortion. While the performance degradation due to tracking error is generally not serious, receiver performance will improve if this tracking error is reduced. The mechanical alignment of the inductors used in this system is also a significant cost item, either in direct labor or investment in automatic alignment equipment.